Sometimes, when a coil of aluminum is unwound, some patches of white, chalky stains can be seen on the surface — this is a sure indication that the coil has been exposed to moisture at some time. Although usually white, the colors of the stains can also be brown, black or even show iridescent colors.

1.How does the staining occur?

Aluminum reacts with oxygen in the air to form a very tenacious oxide coating. It is this coating that gives aluminum its excellent corrosion resistance.

Under most conditions aluminum will not react with water at all, but aluminum is very prone to waterstaining when water is trapped between mating surfaces, such as when it is in the form of a tightly wound coil or in a stack of flat sheet.

Because oxygen from the air is prevented from reaching much of the aluminum surface, a chemical reaction occurs between the entrapped water and the aluminum, which results in a white hydroxide film forming instead of the usual transparent oxide film. The stains have no significant affect on the mechanical strength, but they can be unsightly and are often objectionable for esthetic reasons. They may cause processing problems where additional surface finishing or fabrication is to be performed.

 2.How does the water get in between the wraps?

It is often a source of amazement that waterstaining can occur all the way across a wide strip despite only the edges of a coil getting wet. However the water is forced into the wraps of the coil by a strong force called capillary action (the very tiny gaps between the wraps of the coil cause it to behave like a sponge soaking up water).

3.How to deal with the waterstain?

If there is evidence that the package has been exposed to moisture when you receive the metal, then this should be noted on the receiving papers and you should notify us.

If water is already in contact with the metal, the only sure way to avoid waterstain is to process the metal immediately. If this is not practical then remove the water as quickly as possible by using fans to blow air over the metal. Do not use hot air as this can cause further condensation to occur.